1. Field of the Invention
This invention relates to a semiconductor device and a method of manufacturing the same, and more particularly relates to a semiconductor device including semiconductor layers which are placed on a substrate and are harder than the substrate, and a method of manufacturing such a semiconductor device.
2. Description of the Related Art
A semiconductor device including a GaN-group electronic device with excellent high frequency performance, more particularly an HEMT (high electron mobility) transistor, is well-known as disclosed in Japanese Patent Publication 1991-003936. The HEMT includes a device function layer, a pair of ohmic (low resistance) electrodes, and a gate electrode.
The device function layer is constituted by a GaN layer and an aluminum-gallium-nitride (AlGaN) layer placed on the GaN layer. With the device function layer, a two-dimensional electron gas layer (2DEG) is produced near an interface between the AlGaN layer and the GaN layer. The 2DEG layer functions as a channel region. The pair of ohmic electrodes are placed on the AlGaN layer of the device function layer, and are used as a source electrode and a drain electrode. The gate electrode is in Schottky junction on the AlGaN layer of the device function layer between the ohmic electrodes, and controls a flow of electrons in the two-dimensional electron gases.
A main semiconductor layer including the device function layer of the HEMT is stacked on the substrate. Usually, the main semiconductor layer is formed on the substrate by the epitaxial growth. A GaN substrate, a silicon-carbide (SiC) substrate or a sapphire substrate is used as the substrate for the main semiconductor layer. The GaN substrate is optimum for the growth of the main semiconductor layer mainly made of a nitride group semiconductor. The SiC or sapphire substrate is used assuming that a buffer layer is present on the substrate side of the main semiconductor layer made of a nitride group semiconductor.
Recently, it is expected that a single-crystal silicon (Si) substrate will be practically used for a wafer in a substrate manufacturing process of the foregoing semiconductor device. The use of the Si substrate is effective in reducing a manufacturing cost, enlarging the substrate, and improving device performance.
The semiconductor devices including the HEMT and the manufacturing method thereof seem to suffer from the following problems.
In order to manufacture a plurality of semiconductor devices (semiconductor chips) at one time, the GaN layer and the AlGaN layer are sequentially placed on a buffer layer which is made of a nitride group semiconductor layer and extends over an Si wafer, thereby obtaining the device function layer. The buffer layer, GaN layer and AlGaN layer undergo the epitaxial growth. Thereafter, the ohmic electrodes and gate electrode are formed on the AlGaN layer. Then, the Si wafer, buffer layer on the Si wafer, GaN layer and AlGaN layer are subject to the scribing (or dicing). The Si wafer is cut and is segmented into semiconductor devices. The GaN layer and AlGaN layer are used as the device function layer in the segmented semiconductor devices.
In the segmenting process, the nitride group semiconductor (i.e. the main semiconductor layer) constituting the buffer layer and the device function layer is however harder than the Si wafer, so that side surfaces of the main semiconductor layer of the segmented semiconductor chips tend to crack. Especially, when a rear surface of the semiconductor device (i.e. a rear surface of the Si wafer) and the source electrode are electrically connected, a high voltage should be kept in a depth direction (longitudinally) of the device function layer. This leads to leakage or collapse of currents because of cracks on the side surface of the device function layer.